By purchasing Medivation, Pfizer would add to its portfolio the drug Xtandi, a promising treatment against prostate cancer that analysts estimate will generate some $1.33 billion in annual sales by 2020.

The purchase would be Pfizer's biggest buy since it bought the medical device company Hospira last year for $17 billion.

The transaction is a snub to the French pharmaceutical group Sanofi, which earlier offered $58 a share, which Medivation rejected in July.